The object of the present invention is to produce non-toxic, crystallizable enamels for the decoration of glass-ceramics having low coefficients of thermal expansion.
Glass-ceramics have been employed in the fabrication of culinary ware for about two decades. The glass compositions utilized for that purpose have demonstrated a refractory character and a low coefficient of thermal expansion, this second property imparting good thermal shock resistance to the materials. Continuing research in the area of glass-ceramics for the fabrication of culinary ware has involved the development of enamels permitting them to be decorated; the aesthetic appearance of culinary articles being of great importance to the purchaser.
The origin of the recourse to enamels and glazes for decorating ceramic articles goes back to antiquity. (One commonly defines a "glaze" as being a transparent glass, and an "enamel" as a glass containing particles in suspension.) The classical enamels and glazes employed in decorating culinary articles have relatively high coefficients of thermal expansion (generally ranging between 50-120.times.10.sup.-7 /.degree.C.;) they cannot, therefore, be utilized on glass-ceramics having low coefficients of thermal expansion without crazing or spalling.
Numerous researchers have sought to find enamels and glazes applicable to highly refractory glass-ceramics with a low coefficient of thermal expansion. In this respect it is possible to cite the state of the prior art:
U.S. Pat. No. 3,418,156 describes glazes having high coefficients of thermal expansion but which can be applied to glass-ceramic substrates having a coefficient of thermal expansion below 25.times.10.sup.-7 /.degree.C. over the range of 0.degree.-300.degree. C.
The composition of the glazes, expressed in weight percent on the oxide basis, comprises primarily:
PbO: 35-60 PA1 SiO.sub.2 : 7.5-30 PA1 Cr.sub.2 O.sub.3 : 0.2-19 PA1 B.sub.2 O.sub.3 : 3-12 PA1 Al.sub.2 O.sub.3 : 0.3-7 PA1 SiO.sub.2 : 50-70 PA1 B.sub.2 O.sub.3 : 10-25 PA1 Al.sub.2 O.sub.3 : 8-18 PA1 Li.sub.2 O: 0.1-10 PA1 TiO.sub.2 : 0.1-6 PA1 Bi.sub.2 O.sub.3 : 0.1-6 PA1 SiO.sub.2 : 40-67 PA1 B.sub.2 O.sub.3 : 0.1-10 PA1 Al.sub.2 O.sub.3 : 17-31 PA1 Li.sub.2 O: 3-13 PA1 TiO.sub.2 : 0.1-9 PA1 PbO: 13-23 PA1 B.sub.2 O.sub.3 : 4-11 PA1 Li.sub.2 O: 4-6 PA1 Al.sub.2 O.sub.3 : 14-19 PA1 SiO.sub.2 : 39-50 PA1 PbO: 62-68 PA1 TiO.sub.2 : 12-16 PA1 SiO.sub.2 : 14-20 PA1 B.sub.2 O.sub.3 and/or ZnO and/or BaO and/or P.sub.2 O.sub.5 : 0-2 PA1 Al.sub.2 O.sub.3 : 2-4 PA1 SnO.sub.2 : 0-4 PA1 SiO.sub.2 : 37-49 PA1 Al.sub.2 O.sub.3 : 4-7 PA1 B.sub.2 O.sub.3 : 6.5-11 PA1 ZrO.sub.2 : 0-1.75 PA1 Na.sub.2 O: 1.75-3.5 PA1 K.sub.2 O: 0.75-2.5 PA1 PbO: 21-32 PA1 CdO: 0-0.75 PA1 CaO: 1-9 PA1 TiO.sub.2 : 0-0.35 PA1 F: 0-0.6 PA1 CaO: 10-18 PA1 Al.sub.2 O.sub.3 : 29-35 PA1 SiO.sub.2 : 30-39 PA1 TiO.sub.2 : 13-20 PA1 SiO.sub.2 : 36-58 PA1 Al.sub.2 O.sub.3 : 8.5-16 PA1 PbO: 27-40 PA1 K.sub.2 O: 0.5-6 PA1 CaO: 0.5-10 PA1 SiO.sub.2 : 36-58 PA1 Al.sub.2 O.sub.3 : 9-14 PA1 CaO: 2-8 PA1 PbO: 28-35 PA1 TiO.sub.2 and/or ZrO.sub.2 : 0.5-1.5 or PA1 P.sub.2 O.sub.5 : 0.1-0.7 PA1 K.sub.2 O: 1-3 PA1 Li.sub.2 O: 1-2.5
The mechanism to avoid the formation of cracking and/or spalling during the firing of the glaze is not known with certainty. It is supposed that a reaction takes place at the interface between the glaze and the glass-ceramic that modifies the crystallization of the glass-ceramic and increases the bonding force between the glaze and the glass-ceramic substrate.
U.S. Pat. No. 3,428,466 describes enamels that crystallize when they are applied and fired onto glass-ceramic substrates having low coefficients of thermal expansion. In these enamels the crystallized phase is beta-spodumene and the coefficient of thermal expansion of the enamel is comparable to that of the glass-ceramic substrates. The composition of the enamel, expressed in weight percent on the oxide basis, consisted essentially of:
U.S. Pat. No. 3,463,647 describes crystallizable enamels for glass-ceramic substrates of low coefficient of thermal expansion. The crystallized phase in the enamel is beta-eucryptite; the coefficient of thermal expansion of the enamel is therefore in accord with that of the substrate. The composition of the enamel, expressed in weight percent on the oxide basis, consists essentially of:
U.S. Pat. No. 3,503,763 describes compositions of devitrifiable glasses for solder glasses and coatings; the crystalline phases here are beta-spodumene or beta-eucryptite. The compositions of these enamels, expressed in weight percent on the oxide basis, consist essentially of:
and these crystallized materials have a coefficient of thermal expansion of between -5 and 30.times.10.sup.-7 /.degree.C.
U.S. Pat. No. 3,663,244 describes thermally devitrifiable enamels for decorating or protecting glass-ceramic articles; those enamels having a coefficient of thermal expansion between about 15-30.times.10.sup.-7 /.degree.C. The base composition of those enamels, expressed in weight percent on the oxide basis, consists of:
The crystallizable phase here is lead titanate when the enamel is fired between 700.degree.-850.degree. C.
U.S. Pat. No. 4,331,768 describes glazes having relatively high coefficients of thermal expansion suitable for decorating glass-ceramic culinary ware. Crystals of cassiterite develop during firing of the glaze; the composition of those glazes, expressed in weight percent on the oxide basis, consisting primarily of:
An enamel or glass utilized for the fabrication of cookware or tableware should not release toxic substances when in use. Lead and/or cadmium are often employed in the manufacture of enamels or glazes because they permit the obtaining of a material with a high index of refraction and/or because they improve the flow of the enamel during firing. However, these two elements are extremely toxic; the glaze or enamel must, therefore, exhibit good resistance to chemical attack caused by food products.
It is possible to produce transparent or opaque glass-ceramics, depending upon the composition of the glass and/or the heat treatment applied to it. Glass-ceramics are normally transparent when the crystals they contain are smaller than the wavelength of visible light and/or because they exhibit very little birefringence and their index of refraction is close to that of the residual glass phase.
The compositions of the crystal phases generally present in those glass-ceramics belong to the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system; the most frequent are solid solutions of .beta.-eucryptite and .beta.-quartz. Some examples of compositions of transparent glass-ceramics are described in the following patents:
U.S. Pat. No. 3,157,522 describes the fabrication of transparent glass-ceramics obtained through the thermal treatment of glass having compositions belonging to the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 quaternary; some minor constituents such as Na.sub.2 O, K.sub.2 O, and B.sub.2 O.sub.3 have been added thereto to soften the glass; the principal crystal phase therein being .beta.-eucryptite.
U.S. Pat. No. 3,252,811 describes the fabrication of transparent glass-ceramics in which the major crystal phase is a solid solution of .beta.-quartz. The composition of the base glass belongs to the XO--Al.sub.2 O.sub.3 --SiO.sub.2 system and ZrO.sub.2 is utilized as the nucleating agent (XO designates Li.sub.2 O+ZnO and/or MgO).
U.S. Pat. No. 3,241,985 describes transparent glass-ceramics in which the principal crystal phase is .beta.-eucryptite. The composition of the glass belongs to the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system and the nucleation agent is ZrO.sub.2. Small quantities of alkali metal oxides, alkaline earth metal oxides, and/or TiO.sub.2 may be added to the composition.
U.S. Pat. No. 3,282,712 describes the fabrication of transparent glass-ceramics wherein the major crystal phase is .beta.-eucryptite. The compositions belong to the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 --P.sub.2 O.sub.5 field, the nucleation agents being ZrO.sub.2 and TiO.sub.2. Small quantities of alkaline earth metal oxides may be introduced into the composition.
U.S. Pat. No. 3,484,327 describes obtaining transparent glass-ceramics containing .beta.-eucryptite, produced in the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system and nucleated by TiO.sub.2 +ZrO.sub.2. Alkali metal oxides, alkaline earth metal oxides, and B.sub.2 O.sub.3 may advantageously be added.
U.S. Pat. No. 3,499,773 relates to transparent glass-ceramics containing .beta.-eucryptite as the principal crystal phase produced in the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system, the nucleation agents being TiO.sub.2 and/or ZrO.sub.2 and/or SnO.sub.2. Additions of small quantities of alkali metal oxides, alkaline earth metal oxides, and P.sub.2 O.sub.5 may be included in the composition.
U.S. Pat. No. 3,677,785 describes compositions in the system Li.sub.2 O--BaO--MgO--Al.sub.2 O.sub.3 --SiO.sub.2 with TiO.sub.2 and ZrO.sub.2 as nucleating agents. Addition of small amounts of alkali metal oxides and/or B.sub.2 O.sub.3 may be made.
U.S. Pat. No. 3,788,865 relates to the production of glass-ceramic articles in which .beta.-eucryptite is the major crystal phase obtained by heat treating glasses having compositions belonging to the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system nucleated by TiO.sub.2 and/or ZrO.sub.2 and/or SnO.sub.2. Addition of alkaline earth metal oxides, P.sub.2 O.sub.5 and B.sub.2 O.sub.3 as well as conventional coloring agents are possible.
U.S. Pat. No. 4,018,612 describes transparent glass-ceramics having .beta.-quartz solid solution as the major crystal phase produced from a glass in the Li.sub.2 O--MgO--ZnO--Al.sub.2 O.sub.3 --SiO.sub.2 system nucleated with TiO.sub.2 +ZrO.sub.2. Minor additions of BaO and conventional colorants are proposed.
U.S. Pat. No. 4,093,468 covers compositions of transparent and colorless glass-ceramics in the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system nucleated by TiO.sub.2 ; the major crystal phase is .beta.-quartz solid solution. The introduction of small quantities of neodymium oxide permits obtaining colorless glass-ceramics.
U.S. Pat. No. 4,211,820 describes the production of a transparent glass-ceramic having .beta.-spodumene as the principal crystal phase. On the surface of the glass-ceramic is a thin film having .beta.-quartz solid solution as the principal crystal phase. The composition of the base glass belongs to the Li.sub.2 O--MgO--ZnO--Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 system to which V.sub.2 O.sub.5 is added to obtain a brown coloration.
On another plane, U.S. Pat. No. 4,187,155 describes highly crystalline glass-ceramics having anorthite as the principal crystal phase. The compositions of the glasses of that invention, expressed in weight percent on the oxide basis, belong to the following area.
Up to 10% total of oxides such as MgO, SrO, BaO, PbO, CdO, P.sub.2 O.sub.5, and F may be included within the following limits: up to 3% MgO, up to 10% SrO, up to 5% BaO, up to 7% PbO, up to 5% CdO, up to 6% P.sub.2 O.sub.5, and up to 1.5% F.
But such materials are too crystalline to serve as enamels with good flow.